Environmental Enrichment vs. Cognitive Enrichment in Aged Rats:

1. Environmental Enrichment vs. Cognitive Enrichment in Aged Rats: Effects on Cognition and Hippocampal Neurogenesis Judy C. Songrady & Aileen Bailey, Ph.D Psychology Department, St. Mary’s College of Maryland Introduction Results Environmental enrichment conditions in animals involve a changing environment with several different objects (i.e. wheel, tubing, toys, mini houses, ladders platforms etc.) which are rotated on a regular basis1,2,3,4. Cognitive enrichment can be achieved by training animals in tasks that are cognitively challenging2,4. Environmental enrichment enhances cognition in aged rats, and can produce neurogenesis in the hippocampus1,3,4. Research has also demonstrated that training in various learning tasks (cognitive enrichment) can improve cognition and cause increased neurogenesis in the hippocampus of young and adult rats2,5. Research had not yet directly evaluated animals of any age on different forms of enrichment to see if there are differences in plasticity or future cognitive outcome. Thus, the purpose of this study was to explicitly examine levels of neurogenesis and performance on novel cognitive tasks (Morris Water Maze and Novel Object Recognition) following either cognitive or environmental enrichment in aged rats. Environmentally enriched rats were hypothesized to have increased levels of neurogenesis in the hippocampus and increased cognitive performance in comparison to control rats. Additionally, cognitive enrichment was hypothesized to increase both cognitive performance and neurogenesis over that of control animals. • Figure 1. • A 3 x 5 mixed ANOVA found a significant decrease in latency across the Days, F(4,60) = 18.691; p < .001. • Post-hoc paired samples t-tests showed a significant decrease in latency from day 1 to 2 t(17) = 3.985; p = .001. Additionally significant decreases in latency was found from days 1 to 3, 1 to 4, 1 to 5, 2 to 4, and 2 to 5 p < .005. • There was no effect of Group, F(2,15) = 2.21; p = .14. • There was no significant interaction, F(8,60) = .746, p = .65. • Figure 2. • A 3 x 2 mixed ANOVA found a significant effect for Group, F(2,15) = 4.795; p = .025. • Post-hoc tests with Bonferroni adjusted p < .016 showed the CE group spent significantly more time in the target quadrant on probes 1 and 2 than the CG, t(10) = 4.073; p = .002, t(10) = 3.727; p = .004. • Effect of probe trials came close to significance F(1,15) = 4.465, p = .052. • No significant interaction was found F(2,15) = .529, p = .600. Methods Subjects~Eighteen male Spauge Dawly rats were used, and were approximately 300-360 days old when enrichment began. They were housed on a 12:12 dark-light cycle and maintained in a temperature controlled room. All procedures were approved by the SMCM IACUC committee. Rats were randomly assigned to one of three groups; environmental enrichment (EE), cognitive enrichment (CE), or the control group (CG). Environmental Enrichment~ Each rat was housed individually and received a constant toy in the cage which was rotated daily. Rats were also individually rotated into a large cage (30"L x 18"W x 16"H) one at a time. The large cage contained a running wheel, tubes, a platform, and a mini house. EE was maintained until the CE group completed their training. Cognitive Enrichment~ Subjects were trained in an operant box on a set-shifting task thought to be cognitively demanding. The set-shifting task required an average of 10 days to complete. Measurement of Neurogenesis~ 5-bromo-2-deoxyuridine (BrdU) is a chemical that integrates itself into the neuronal structure during cell birth and is commonly used among researchers to label new neurons found in the hippocampus1. Subjects received three injections of BrdU during cognitive training to measure neurogenesis in the hippocampus. Injections (40mg/kg – i.p.) were given on day (30 min prior) of original rule learning and on the cognitive shift day. All other animals (EE,CG) received injections of the same concentration on the same day as CE group. Morris Water Maze~Subjects were trained for 5 days with 4 trials per day. Platform remained in a fixed location. Rats started from a new location every trial and were reliant on four spatial cues to navigate. For each in which trial latency to platform was recorded. Probe trials were also conducted on day 2 and 5; time spent in designated platform section was recorded. Novel Object Recognition~ Subjects were allowed to explore two different objects during the first 5 min. session. An hour later rats were given a second 4 min. session in which one of the objects was switched with a novel object. Percent of time spent exploring the novel object was recorded. Histological Methods~ Rats were perfused and brains were sectioned through the rostral-caudal extent of the hippocampus and stained for BrdU positive cells. BrdU levels were measured via Image J software. • Figure 3. • A one-way between-subjects ANOVA found no difference between the groups on the percentage of time spent with the novel object, F(2,15) = 1.40; p = .28. • Figure 4. • A one-way between subjects ANOVA found no difference in overall hippocampal density between the three groups, F(2,15) = .06; p = .94. • Figure 5. • A 3 x 3 mixed ANOVA found no effect of Group, F(2,9) = .113; p = .89. • There was no effect of Region, F(2,18) = .037, p = .96 • A significant interaction was found, F(4,18) = 4.035, p = . 017 EE (Rat 1)-Ant CE (Rat 11)-Ant CG (Rat 16)-Ant Conclusions • Morris Water Maze trials showed that there was no differences spatial cognition between the groups. Subjects preformed equivalently across the training days. Probe Trials in the water maze did reveal a difference in performance between the subjects. CE preformed significantly better than CG, however there was no differences between EE and CG groups or between enriched groups. • The Novel Object task corresponds to the results of the water maze trials in that there was no significant difference between the EE, CE, and CG groups and their spatial working memory of a novel or old object. • BrdU histology found no difference between the groups in the number of newly generated neurons in the hippocampus. This may be a result of the methodologies used to count the neurons. Research suggests that finding the BrdU labeled cells is extremely difficult and time consuming. • Probe tests measure spatial cognition, whereas novel object recognition measures working memory function. Results suggest a possibility that cognitive enrichment does not effect cognition and memory equivalently. • This research can contribute to our understanding of the aging brain by demonstrating how different enrichment experiences may improve cognition. In this case the results suggest that there is an effect of cognitive enrichment on spatial reference memory. Environmental Enrichment References 1. Bruel-Jungerman, E., Laroche, S., & Rampon, C. (2005, January). New neurons in the dentate gyrus are involved in the expression of enhanced long-term memory following environmental enrichment. European Journal of Neuroscience, 21(2), 513-521. 2. During, M., & Cao, L. (2006, February). VEGF, a Mediator of the Effect of Experience on Hippocampal Neurogenesis. Current Alzheimer Research, 3(1), 29-33. 3. Iso, H., Simoda, S., & Matsuyama, T. (2007). Environmental change during postnatal development alters behaviour, cognitions, and neurogenesis of mice. Behavioral Brain Research, 179, 90-98. 4. Kempermann, G., Kuhn, H., & Gage, F. (1998, May). Experience-induced neurogenesis in the senescent dentate gyrus. Journal of Neuroscience, 18(9), 3206-3212. 5. Lores-Arnaiz, S., Bustamante, J., Arismendi, M., Vilas, S., Paglia, N., Basso, N., et al. (2006, April). Extensive enriched environments protect old rats from the aging dependent impairment of spatial cognition, synaptic plasticity and nitric oxide production. Behavioural Brain Research, 169(2), 294-302. Acknowledgements I would like to dedicate this space to give a special thanks to Dr. Aileen Bailey, Dr. Anne Marie Brady, Jennifer St. Germain, and Angela Draheim for all their help and support.